The conditions of a gate oxide film/silicon substrate interface of a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor; hereinafter referred to as a MOS transistor) formed on a silicon substrate are important, because the conditions influence the properties of the MOS transistor. The presence of carrier traps at a gate oxide film/silicon substrate interface causes problems such as mobility reduction and increase of 1/f noise. Therefore, formation of a gate oxide film/silicon substrate interface that has as few carrier traps as possible is desirable.
Typical carrier traps are interface states. Interface states are states that occur in a forbidden band at a surface of a semiconductor due to uncoupled bonds (i.e. dangling bonds) of crystal atoms. In order to reduce interface states, therefore, dangling bonds need to be terminated.
A typical way to terminate dangling bonds is to combine hydrogen, fluorine atoms, or the like each having only one bond, with dangling bonds. PTL 1 describes a method in which fluorine ions are implanted in a polysilicon film that will serve as a gate electrode and fluorine is introduced into a gate oxide film and a gate oxide film/silicon substrate interface by a subsequent thermal treatment. PTL 2 describes another method in which fluorine ions are implanted into a silicon substrate prior to formation of a gate oxide film and fluorine is introduced in a gate oxide film/silicon substrate interface and in the gate oxide film by thermal treatment during formation of the gate oxide film.